Ordered Hierarchical Porous Structure of PtSn/3DOMM-Al2O3 Catalyst for Promoting Propane Non-Oxidative Dehydrogenation

Herein, the hierarchical porous catalyst of 3-dimensional ordered macro-mesoporous (3DOMM) Al2O3 supported active PtSn nanoparticles (NPs) was prepared by the combined synthesized path of evaporation-induced self-assembly with colloid crystal template (EISA-CCT) methods. The hierarchical macro-mesoporous composite structure can markedly increase the specific surface area, accommodate the diffusion of propene, and decrease the number of surface acid sites. In addition, the special surface property and pore structure of 3DOMM-Al2O3 can modify the interaction between metals and substrates, as well as stabilize the metal nanoparticle, which promotes the formation of a highly active and stable PtSn phase. The PtSn/3DOMM-Al2O3 catalyst exhibits higher productivity and stability than PtSn/Al2O3 catalysts with macropore and mesopore structures. The PtSn/3DOMM-Al2O3 catalyst displays the best catalytic performance with propylene selectivity over 95% at a propane conversion of 33.9%. The study of the ordered hierarchical porous structure of PtSn/3DOMM-Al2O3 catalysts can contribute to obtaining improved catalysts in industrial processes.


Introduction
Propylene is a significantly important hydrocarbon compound that contributes to the intermediates of a variety of chemical products and is widely used in human production and life [1]. In recent years, thermal and catalytic cracking technologies, which are the traditional production processes of propylene, have no longer been able to meet the increasing needs of the international market [2,3]. Producing propylene through the propane non-oxidative dehydrogenation (PDH) process has become a promising and effective approach. Furthermore, cheap propane is supplied due to the recently increased exploitation of shale gas [4,5]. Although there is keen industrial interest in PDH technology, the dehydrogenation reaction still needs to operate at a high temperature of about 650 • C to break the stable C-H bond of alkane [6,7]. The harsh reaction condition could cause many problems for Pt-based catalysts, such as coke deposition and the sintering of metal nanoparticles [8]. The development of Pt-based catalysts with improved yield, selectivity, and stability is the key issue in improving the industrial utilization value of PDH reactions.
Although the Pt element reveals high activity in breaking the C-H bond, the Pt-based catalysts without any promoters still have a high tendency to side reactions such as hydrogenolysis, cracking, and coke deposition and sintering during the reaction process [9][10][11][12]. Comparing pure Pt-based catalysts, the selectivity and stability of PDH can be significantly improved in PtSn-based catalysts. The Sn element could be reduced by hydrocarbon and alloyed with Pt to form PtSn nanoparticles. Pt is the active species of the PtSn-based catalyst, while Sn acts as the promoter [13][14][15][16]. The structural effects and electronic interaction of

Catalyst Preparation
PtSn/γ-Al 2 O 3 catalysts with ordered mesopore (M-Al 2 O 3 ), 3-dimensional ordered macropore (3DOM-Al 2 O 3 ), and 3-dimensional ordered macro-mesopore (3DOMM-Al 2 O 3 ) were obtained by varied methods. Polymethyl methacrylate (PMMA) microspheres were tightly packed three-dimensionally (3D) after the centrifugal operation. As shown in Figure S1, the particle size of the microspheres is around 300 nm. These 3D close-packed microspheres were used to obtain the macropore structure. The PMMA microspheres were immersed in an Al precursor solution. The solution fills the interspace between the PMMA microspheres to build structure of ordered macropores. F127 was employed to obtain a mesoporous structure. As the solvent evaporates, F127 molecules tend to form saturated micelles, which arrange themselves into a two-dimensional hexagon (p6mm) ordered pattern. During the growth process, Al ions adhere to the template to build ordered mesopores. After drying and calcining, the template (F127 and PMMA) can be removed and the ordered pores (macropore and mesopore) structure is obtained. As shown in Scheme 1, the 3DOMM nanostructured catalyst was obtained by the EISA-CCT method using two templates, while the 3DOMM structure was obtained after drying and calcining. For all the catalysts, the templates (F127 micelles or PMMA microspheres) were removed in a tube furnace under air (50 mL min −1 ) at 300 • C for 3 h, and then the temperature was increased to 650 • C for 4 h to crystallize the alumina. After obtaining γ-Al 2 O 3 with different morphologies, Pt species (0.5 wt%) and Sn species (1 wt%) were added into the support by incipient wetness impregnation to obtain the PtSn/γ-Al 2 O 3 catalyst. A detailed description of the synthesis pathway is listed in part of the Supporting Information.
were obtained by varied methods. Polymethyl methacrylate (PMMA) microspheres were tightly packed three-dimensionally (3D) after the centrifugal operation. As shown in Figure S1, the particle size of the microspheres is around 300 nm. These 3D close-packed microspheres were used to obtain the macropore structure. The PMMA microspheres were immersed in an Al precursor solution. The solution fills the interspace between the PMMA microspheres to build structure of ordered macropores. F127 was employed to obtain a mesoporous structure. As the solvent evaporates, F127 molecules tend to form saturated micelles, which arrange themselves into a two-dimensional hexagon (p6mm) ordered pattern. During the growth process, Al ions adhere to the template to build ordered mesopores. After drying and calcining, the template (F127 and PMMA) can be removed and the ordered pores (macropore and mesopore) structure is obtained. As shown in Scheme 1, the 3DOMM nanostructured catalyst was obtained by the EISA-CCT method using two templates, while the 3DOMM structure was obtained after drying and calcining. For all the catalysts, the templates (F127 micelles or PMMA microspheres) were removed in a tube furnace under air (50 mL min −1 ) at 300 °C for 3 h, and then the temperature was increased to 650 °C for 4 h to crystallize the alumina. After obtaining γ-Al2O3 with different morphologies, Pt species (0.5 wt%) and Sn species (1 wt%) were added into the support by incipient wetness impregnation to obtain the PtSn/γ-Al2O3 catalyst. A detailed description of the synthesis pathway is listed in part of the Supporting Information. Scheme 1. Diagram for the preparation of the 3DOMM-Al2O3 support.

Characterization
Scanning electron microscope (SEM) graphs were observed using a ZEISS Gemini SEM 300 microscope. Transmission electron microscope (TEM) images were obtained and energy dispersive spectrometer (EDS) element mapping was performed using the LaB6 2100 electron microscope.
The N2 adsorption-desorption results were obtained using Micromeritics TriStar-Ⅱ 3020 equipment. The catalyst was pretreated at 200 °C in a constant temperature tank to remove the adsorbed vapor. The X-ray diffraction (XRD) pattern was investigated with a Bruker Advance spectrometer.
The 27 Al nuclear magnetic resonance (NMR) spectra were obtained using an Agilent 600 DD2 spectrometer. The temperature-programmed desorption of NH3 (NH3-TPD) was recorded using Autochem-II 2920 equipment. The catalyst (0.1 g) was placed in a quartz reactor. The catalyst was pretreated with He at 300 °C for 1 h and cooled to 50 °C, then Scheme 1. Diagram for the preparation of the 3DOMM-Al 2 O 3 support.

Characterization
Scanning electron microscope (SEM) graphs were observed using a ZEISS Gemini SEM 300 microscope. Transmission electron microscope (TEM) images were obtained and energy dispersive spectrometer (EDS) element mapping was performed using the LaB6 2100 electron microscope.
The N 2 adsorption-desorption results were obtained using Micromeritics TriStar-II 3020 equipment. The catalyst was pretreated at 200 • C in a constant temperature tank to remove the adsorbed vapor. The X-ray diffraction (XRD) pattern was investigated with a Bruker Advance spectrometer.
The 27 Al nuclear magnetic resonance (NMR) spectra were obtained using an Agilent 600 DD2 spectrometer. The temperature-programmed desorption of NH 3 (NH 3 -TPD) was recorded using Autochem-II 2920 equipment. The catalyst (0.1 g) was placed in a quartz reactor. The catalyst was pretreated with He at 300 • C for 1 h and cooled to 50 • C, then treated with 10% NH 3 balanced with He (30 mL min −1 ) for 1 h and then purged with pure He (30 mL min −1 ) for 1 h. The temperature was raised to 700 • C with a heating rate of 10 • C min −1 . The NH 3 desorption signal was observed using a thermal conductivity detector (TCD). The pyridine infrared adsorption (Py-IR) results were given by the Nicolet 6700 Fourier Transform Infrared spectrometer. Before the test, the catalysts were dried for 1 h at 300 • C.
The temperature-programmed reduction of the H 2 (H 2 -TPR) pattern was conducted on a homemade fixed-bed reactor. The catalyst (0.1 g) was dried at 300 • C for 1 h, and then cooled to 50 • C under N 2 . Then, the H 2 balanced by N 2 (10%, 50 mL min −1 ) went through the bed at a temperature of 50-750 • C with a heating rate of 10 • C min −1 . The consumption signal was recorded by a TCD. The temperature-programmed desorption of H 2 (H 2 -TPD) was conducted using similar equipment with H 2 -TPR. The catalyst (0.1 g) was first dried for 1 h at 300 • C and reduced in flowing H 2 (pure, 30 mL min −1 ) at 600 • C for 2 h. Then, the catalyst was cooled to room temperature and adsorbed in H 2 for 1 h. The temperature was increased to 700 • C with a heating rate of 10 • C min −1 and the desorption signal was recorded by the TCD.
The differential thermal analysis and thermal gravimetric analysis (TG-DTA) were conducted in a homemade reaction instrument. The temperature was increased from room temperature to 800 • C at a rate of 10 • C min −1 under air (30 mL min −1 ). Raman patterns were obtained with Renishaw inVia reflex Raman equipment with excitation wavelengths of 532 nm.

Evaluation of Catalytic Performance
The PDH tests were conducted in a quartz tubular micro-reactor. The catalyst (0.2 g, 40-60 meshes) was loaded into the reactor. Before the catalytic evaluation, the catalyst was pretreated with H 2 (pure, 10 mL min −1 ) for 2 h at 500 • C. The PDH reaction was carried out at 600 • C. The propane was fed at a weight hourly space velocity (WHSV) of 3.0 h −1 (C 3 H 8 /H 2 = 1/1). Then, the products were analyzed using online gas chromatography (GC) equipped with a flame ionization detector (FID) and a TCD. The conversion and selectivity were obtained as follows:

Physical Properties and Morphology
The morphology of the PMMA microspheres and macropore catalysts was identified by SEM. As shown in Figure S1, the PMMA microspheres are closely packed and form a uniform interspace between them. Tightly arranged and uniform PMMA microspheres are important for the formation of ordered macropores. During the EISA-CCT process, Al ions hydrolyze, condense, and crystallize in the interspace of PMMA microsphere templates to form a macropore structure. As shown in Figure 1A,B, the PtSn/3DOM-Al 2 O 3 and PtSn/3DOMM-Al 2 O 3 catalysts exhibit well-defined and uniform 3D-ordered macropore structures. The average diameter of the macropore (about 240 nm) is shrunken compared with that of the PMMA microsphere templates (about 300 nm), which can be attributed to the melting of the PMMA microspheres and the crystallization of the Al 2 O 3 during the calcination process [26]. Notably, some areas in the macropore also display disorder and a broken structure, which can be attributed to the mechanical milling which will inevitably damage part of the porous structure during the preparation of the SEM sample. The mesoporous structures and supported metal nanoparticles (NPs) were identified with a TEM. As shown in Figure 2A,B, the PtSn/3DOMM-Al2O3 sample shows an ordered macropore structure. The ordered macropores are approximately 220 nm in pore size, and the thickness of the Al2O3 wall is approximately 80 nm, which matches the SEM image. In addition, there are many ordered mesopores present on the macropore wall of the PtSn/3DOMM-Al2O3 catalyst ( Figure 2C). Ordered mesopores with a diameter of about 8-9 nm are densely arranged in a hexagon region on the macropore wall of 3DOMM Al2O3,  The mesoporous structures and supported metal nanoparticles (NPs) were identified with a TEM. As shown in Figure 2A,B, the PtSn/3DOMM-Al 2 O 3 sample shows an ordered macropore structure. The ordered macropores are approximately 220 nm in pore size, and the thickness of the Al 2 O 3 wall is approximately 80 nm, which matches the SEM image. In addition, there are many ordered mesopores present on the macropore wall of the PtSn/3DOMM-Al 2 O 3 catalyst ( Figure 2C). Ordered mesopores with a diameter of about 8-9 nm are densely arranged in a hexagon region on the macropore wall of 3DOMM Al 2 O 3 , indicating the hierarchical macro-mesoporous composited structure is successfully synthesized by the EISA-CCT method. The highly dispersed metal nanoparticles are formed on the surface of 3DOMM-Al 2 O 3 with a uniform particle size of around 3 nm ( Figure 2D). The atomic number of the Pt element is much larger than the Al element. Therefore, the metal NPs can be attributed to the Pt or PtSn alloy, while the nearly colorless parts can be attributed to the mesopores. For identifying the distribution of metal on the support, an EDS was employed. The elemental mapping images show Pt and Sn elements evenly distributed on the surface of the catalysts, indicating the formation of a PtSn alloy. In addition, TEM images of the PtSn/3DOM-Al 2 O 3 and PtSn/M-Al 2 O 3 catalysts are shown in Figure S2. The PtSn/M-Al 2 O 3 catalyst has a mesopore structure with an average size of 10 nm, while the PtSn/3DOM-Al 2 O 3 catalyst only has a macropore structure. The mesoporous structures and supported metal nanoparticles (NPs) were identified with a TEM. As shown in Figure 2A,B, the PtSn/3DOMM-Al2O3 sample shows an ordered macropore structure. The ordered macropores are approximately 220 nm in pore size, and the thickness of the Al2O3 wall is approximately 80 nm, which matches the SEM image. In addition, there are many ordered mesopores present on the macropore wall of the PtSn/3DOMM-Al2O3 catalyst ( Figure 2C). Ordered mesopores with a diameter of about 8-9 nm are densely arranged in a hexagon region on the macropore wall of 3DOMM Al2O3, indicating the hierarchical macro-mesoporous composited structure is successfully synthesized by the EISA-CCT method. The highly dispersed metal nanoparticles are formed on the surface of 3DOMM-Al2O3 with a uniform particle size of around 3 nm ( Figure 2D). The atomic number of the Pt element is much larger than the Al element. Therefore, the metal NPs can be attributed to the Pt or PtSn alloy, while the nearly colorless parts can be attributed to the mesopores. N2 adsorption-desorption experiments were applied to obtain the textual structure of catalysts. As shown in Figure 3A, the M-Al2O3 support shows an H1 hysteresis loop with the IV-type isotherm, which is characteristic of an ordered mesoporous nanostructure. The M-Al2O3 support has a specific surface area of 347 m 2 g −1 . The 3DOM-Al2O3 support shows the H3 hysteresis loop attributed to the ordered macropore structure. The specific surface area of the PtSn/3DOM-Al2O3 sample is 273 m 2 g −1 . Due to the presence of a hierarchical macro-mesoporous composite structure, the 3DOMM-Al2O3 material possesses the highest specific surface area of 369 m 2 g −1 [27]. After the impregnation process, the specific surface area of the PtSn/3DOMM-Al2O3 sample is still maintained at 346 m 2 with the IV-type isotherm, which is characteristic of an ordered mesoporous nanostructure. The M-Al 2 O 3 support has a specific surface area of 347 m 2 g −1 . The 3DOM-Al 2 O 3 support shows the H3 hysteresis loop attributed to the ordered macropore structure. The specific surface area of the PtSn/3DOM-Al 2 O 3 sample is 273 m 2 g −1 . Due to the presence of a hierarchical macro-mesoporous composite structure, the 3DOMM-Al 2 O 3 material possesses the highest specific surface area of 369 m 2 g −1 [27]. After the impregnation process, the specific surface area of the PtSn/3DOMM-Al 2 O 3 sample is still maintained at 346 m 2 g −1 , indicating that the impregnated metal is evenly dispersed on the support. As shown in Figure 3B, the M-Al 2 O 3 and 3DOMM-Al 2 O 3 supports represent an even pore size distribution from 5 to 10 nm, which can be assigned to the mesopore structure. Furthermore, there is an additional pore distribution above 50 nm for the 3DOM-Al 2 O 3 and 3DOMM-Al 2 O 3 supports, which can be attributed to the macropore structure. The impregnation of PtSn NPs causes an obvious decrease in the pore distribution of mesopores, confirming that PtSn NPs are mainly dispersed in mesopores.
3DOMM-Al2O3 support has a hierarchical macro-mesoporous composite structure, wh the PtSn NPs highly disperse in the mesopores of the PtSn/3DOMM-Al2O3 catalyst.

Surface Property of Support
To perform an in-depth investigation of the surface characteristics of the Al2O3 s port, solid-state 27 Al NMR techniques were employed. Figure 5 exhibits the NMR spec of the PtSn/Al2O3 catalysts. Three resonance signals were observed at 73, 44, and 9 pp which were assigned to the Al 3+ ion in the tetra-coordinated (AlO4), penta-coordina (AlO5), and hexa-coordinated (AlO6) coordination, respectively [29]. Penta-coordina AlO5 is universally acknowledged to be the site for anchoring metal sites due to its co dination of unsaturated properties. For Pt/γ-Al2O3 catalysts, coordination unsatura Wide-angle XRD measurements were employed to investigate the crystal phase of the PtSn/Al 2 O 3 catalysts. As shown in Figure 4A, the PtSn/M-Al 2 O 3 , PtSn/3DOM-Al 2 O 3 , and PtSn/3DOMM-Al 2 O 3 catalysts exhibit characteristic diffraction peaks at 31.9 • , 39.4 • , 45.8 • , and 66.8 • , which are attributed to (220), (222), (400), and (440) facets (PDF#79-1558) of Al 2 O 3 , respectively. No characteristic peaks corresponding to Pt and Sn were observed, which is due to the highly dispersed Pt-Sn NPs. In addition, low-angle XRD was employed to verify the ordered mesoporous nanostructure. As shown in Figure 4B, PtSn/M-Al 2 O 3 and PtSn/3DOMM-Al 2 O 3 catalysts exhibit a diffraction peak of 2θ at 0.6 • , which is attributed to the (100) facet of the 2D hexagonal (p6mm) of the ordered mesopore nanostructure [28]. For the PtSn/3DOM-Al 2 O 3 catalysts, the characterized diffraction peak is not located at the same position, indicating an irregular pore structure. In sum, the 3DOMM-Al 2 O 3 support has a hierarchical macro-mesoporous composite structure, while the PtSn NPs highly disperse in the mesopores of the PtSn/3DOMM-Al 2 O 3 catalyst.

Surface Property of Support
To perform an in-depth investigation of the surface characteristics of the Al2O3 support, solid-state 27 Al NMR techniques were employed. Figure 5 exhibits the NMR spectra of the PtSn/Al2O3 catalysts. Three resonance signals were observed at 73, 44, and 9 ppm, which were assigned to the Al 3+ ion in the tetra-coordinated (AlO4), penta-coordinated (AlO5), and hexa-coordinated (AlO6) coordination, respectively [29]. Penta-coordinated AlO5 is universally acknowledged to be the site for anchoring metal sites due to its coordination of unsaturated properties. For Pt/γ-Al2O3 catalysts, coordination unsaturated penta-coordination (AlO5) is the main site for stabilizing Pt [30]. The PtSn/3DOMM-Al2O3 catalyst has a much greater proportion of penta-coordinated Al 3+ ions on its surface than both the PtSn/M-Al2O3 and PtSn/3DOM-Al2O3 catalysts. In addition, the proportions of Al 3+ ions with different coordination sites are exhibited in Table 1. The PtSn/3DOMM-Al2O3 catalyst has the highest fraction of penta-coordinated Al 3+ ions at 9%, while the

Surface Property of Support
To perform an in-depth investigation of the surface characteristics of the Al 2 O 3 support, solid-state 27 Al NMR techniques were employed. Figure 5 exhibits the NMR spectra of the PtSn/Al 2 O 3 catalysts. Three resonance signals were observed at 73, 44, and 9 ppm, which were assigned to the Al 3+ ion in the tetra-coordinated (AlO 4 ), penta-coordinated (AlO 5 ), and hexa-coordinated (AlO 6 ) coordination, respectively [29]. Penta-coordinated AlO 5 is universally acknowledged to be the site for anchoring metal sites due to its coordination of unsaturated properties. For Pt/γ-Al 2 O 3 catalysts, coordination unsaturated pentacoordination (AlO 5 ) is the main site for stabilizing Pt [30]. The PtSn/3DOMM-Al 2 O 3 catalyst has a much greater proportion of penta-coordinated Al 3+ ions on its surface than both the PtSn/M-Al 2 O 3 and PtSn/3DOM-Al 2 O 3 catalysts. In addition, the proportions of Al 3+ ions with different coordination sites are exhibited in Table 1

Catalyst
Peak Area (%) Ⅵ Ⅴ Ⅳ  The acid property of the varied catalysts was studied using NH 3 -TPD experiments, and the corresponding NH 3 -TPD patterns are shown in Figure 6. It is clear that two ammonia desorption peaks can be observed in the PtSn/3DOM-Al 2 O 3 catalysts. The     Figure 7 exhibits the IR pattern of the three samples after the adsorption of pyridine (Py). The PtSn/3DOM-Al 2 O 3 catalyst shows a very weak IR peak located at 1550 cm −1 , which can be attributed to the peak of the Brønsted acid site. Moreover, there is no Brønsted acid site in the PtSn/M-Al 2 O 3 and PtSn/3DOMM-Al 2 O 3 catalysts [33]. The Py molecules can be also adsorbed by the Lewis acid site. The IR peak at 1595 cm −1 is due to the Py molecule adsorbed at the weak Lewis acidic site. The IR peak at 1605 cm −1 can be attributed to the Py molecule adsorbed at the Lewis acid site with medium strength. In addition, the IR peak located at 1626 cm −1 can be attributed to strong Lewis acid sites [34].

Structure of Active Sites
The nanopore structure and surface chemical property of catalysts can closely affect the Pt-Sn interaction and dispersion of metal nanoparticles. Therefore, H2-TPR measurements were employed to test the redox property of the PtSn/γ-Al2O3 samples, which is shown in Figure 8. The peak at about 200 °C is the reduction peak of the Pt component. During the reduction, the metallic state (Sn 0 ) can be reduced to form the PtSn alloy, in which Sn can increase the reduction temperature of Pt. All the PtSn/γ-Al2O3 catalysts show a reduction peak located at around 400 °C , which is attributed to the Pt-Sn alloy formation [35,36]. Notably, the reducibility of catalysts is highly dependent on the surface chemical property of the support. The reduction temperatures of the PtSn/M-Al2O3 and PtSn/3DOMM-Al2O3 catalysts are located at 479 and 455 °C , which are higher than the

Structure of Active Sites
The nanopore structure and surface chemical property of catalysts can closely affect the Pt-Sn interaction and dispersion of metal nanoparticles. Therefore, H 2 -TPR measurements were employed to test the redox property of the PtSn/γ-Al 2 O 3 samples, which is shown in Figure 8. The peak at about 200 • C is the reduction peak of the Pt component. During the reduction, the metallic state (Sn 0 ) can be reduced to form the PtSn alloy, in which Sn can increase the reduction temperature of Pt. All the PtSn/γ-Al 2 O 3 catalysts show a reduction peak located at around 400 • C, which is attributed to the Pt-Sn alloy formation [35,36]. Notably, the reducibility of catalysts is highly dependent on the surface chemical property of the support. The reduction temperatures of the PtSn/M-Al 2 O 3 and PtSn/3DOMM-Al 2 O 3 catalysts are located at 479 and 455 • C, which are higher than the reduction temperature of the PtSn/3DOM-Al 2 O 3 catalyst (420 • C). This increased reduction temperature is attributed to a stronger interaction between the metal and the support, which infers a higher dispersion of metal species. In addition, the PtSn/3DOMM-Al 2 O 3 and PtSn/3DOM-Al 2 O 3 catalysts show a reduction peak at a temperature above 500 • C, which is attributed to the biggish SnO x nanoparticle weakly anchored by γ-Al 2 O 3 , indicating that the macropore nanostructure would affect the dispersion of metal. This can be attributed to the varied geometric confining effect of the macropores and mesopores on the loaded species. The macropore structure will lead to an increase in metal particle size and a decreased interaction between the metal and the carrier. For 3DOMM materials, the abundant mesoporous are conducive to the formation of small PtSn NPs through the confining effect of mesopores. overflow the adjacent support, which is also known as hydrogen spillover. The high-temperature desorption peak above 600 °C can be attributed to the desorption peak of spilled H atoms [40]. Overall, the pore structure of γ-Al2O3 strongly affects the dispersion of the PtSn active sites. Strong metal-support interactions (SMSI) and the confined effect of mesopores in the macropore walls can result in small PtSn nanoparticles forming.   In order to analyze the dispersion of the PtSn species more thoroughly, the H 2 -TPD measurement is further employed. The H 2 -TPD patterns of the different samples are shown in Figure 9, while two desorption peaks can be detected in the patterns. It is well known that H 2 could dissociate into two H atoms on the surface of metal active sites. Based on this, the low-temperature desorption peak (below 500 • C) is assigned to chemisorbed H atoms removed from the metal nanoparticles [37][38][39]. For the PtSn/3DOMM-Al 2 O 3 catalyst, it displays the highest amounts of adsorbed hydrogen, which indicates that PtSn nanoparticles are highly dispersed on 3DOMM-Al 2 O 3 after reduction. The PtSn/3DOMM-Al 2 O 3 demonstrates a much larger peak area of H adsorption than the PtSn/3DOM-Al 2 O 3 sample, indicating that the Pt active sites are highly dispersed on the 3DOMM-Al 2 O 3 support. In addition, after chemical adsorption, the H atoms on the surface of Pt can also overflow the adjacent support, which is also known as hydrogen spillover. The hightemperature desorption peak above 600 • C can be attributed to the desorption peak of spilled H atoms [40]. Overall, the pore structure of γ-Al 2 O 3 strongly affects the dispersion of the PtSn active sites. Strong metal-support interactions (SMSI) and the confined effect of mesopores in the macropore walls can result in small PtSn nanoparticles forming.

Catalytic Performance for PDH
The catalytic performances of the PtSn/γ-Al2O3 catalyst are shown in Figure 10A,B, which shows changes in conversion and selectivity with operating time, respectively. Among all the PtSn/Al2O3 catalysts, the PtSn/3DOMM-Al2O3 catalyst shows the most desired catalytic performance. The initial conversion rate of 51.0% reduced to a conversion rate of 33.9% after 4 h of reaction, and the selectivity increased from 90.7% to 96.5%. The

Catalytic Performance for PDH
The catalytic performances of the PtSn/γ-Al 2 O 3 catalyst are shown in Figure 10A,B, which shows changes in conversion and selectivity with operating time, respectively. Among all the PtSn/Al 2 O 3 catalysts, the PtSn/3DOMM-Al 2 O 3 catalyst shows the most desired catalytic performance. The initial conversion rate of 51.0% reduced to a conversion rate of 33.9% after 4 h of reaction, and the selectivity increased from 90.7% to 96.5%. The PtSn/M-Al 2 O 3 catalyst reveals a high initial conversion rate of propane (47.9%) but a low initial selectivity of propylene (initial: 70.9%), and it quickly deactivates within the first hour. However, the conversion rate dropped to 33.0% after 4 h, while the selectivity of propylene gradually increased (final: 90.3%). Although the conversion rate of PtSn/3DOM-Al 2 O 3 is the lowest (initial: 30.4%, final: 14.5%), its selectivity (initial: 85.7%, final 92.3%) is higher than that of PtSn/M-Al 2 O 3 . This phenomenon can be explained by the volcanic curve relationship between selectivity and conversion in a PDH reaction [41]. Compared with PtSn/γ-Al 2 O 3 catalysts with other porous structures, the hierarchical macro-mesoporous PtSn/3DOMM-Al 2 O 3 catalyst has a higher conversion, selectivity, and strong stability. As shown in Figure 10C, it is also obvious that the PtSn/3DOMM-Al 2 O 3 catalyst possesses the highest propylene yield, followed by PtSn/M-Al 2 O 3 , and the PtSn/3DOM-Al 2 O 3 catalysts have the lowest yield during a 4 h reaction test. The PtSn/3DOMM-Al 2 O 3 catalyst displays better comprehensive conversion and selectivity than the PtSn/M-Al 2 O 3 and PtSn/3DOM-Al 2 O 3 catalysts. In addition, the catalytic performance of the PtSn/3DOMM-Al 2 O 3 catalyst was compared with that of Pt-based catalysts reported recently. As shown in Table S1, the PtSn/3DOMM-Al 2 O 3 catalyst has high conversion and selectivity, indicating that it has a good prospect for industrial PDH application.
In order to improve the gas-solid contact area, catalysts for alkane dehydrogenation reactions are often designed with mesoporous structures. However, a small mesoporous pore size will cause the slow desorption of propylene, coke deposition, and blocked pores. For the 3DOMM-Al 2 O 3 support, the PtSn active sites are mainly located in the mesoporous pores of the macropore walls, which effectively improves the pore connectivity and pore volume, facilitates the diffusion of propylene from pores and improves the carbon-holding capacity of the PtSn/γ-Al 2 O 3 catalyst [42]. Therefore, the PtSn/3DOMM-Al 2 O 3 catalyst shows the high stability for the PDH reaction. The specific surface area of the 3DOMM-Al 2 O 3 material is also significantly higher than the specific surface area of the 3DOM-Al 2 O 3 material due to the abundant mesopores in the macropore walls, which is beneficial to the dispersion of PtSn-active species. In addition, the hierarchical macro-mesoporous structure also affects the surface chemical property of γ-Al 2 O 3 . The catalytic dehydrogenation reaction performance of the catalyst is closely related to the chemical property of active metal, while acid sites on γ-Al 2 O 3 are important to side reactions [43]. For the PtSn/M-Al 2 O 3 catalyst, the amount of acid sites is relatively higher than the PtSn/3DOM-Al 2 O 3 and PtSn/3DOMM-Al 2 O 3 catalysts, consequently reducing the reaction selectivity and stability. In addition, for the PtSn/3DOMM-Al 2 O 3 catalyst, the PtSn nanoparticles have a higher dispersion than that of the PtSn/3DOM-Al 2 O 3 catalyst. Through the characterization of H 2 -TPR and H 2 -TPD, it can be concluded the interaction between the 3DOMM-Al 2 O 3 support and PtSn component is beneficial to the formation of ultrafine Pt-Sn alloy particles. The propane dehydrogenation reaction is structurally insensitive, and its side reactions are structurally sensitive. The smaller metal nanoparticles can inhibit side reactions, which also increase the stability of the PtSn/3DOMM-Al 2 O 3 catalyst [44]. Therefore, the PtSn/3DOMM-Al 2 O 3 catalyst shows the best catalytic performance among all the PtSn/γ-Al 2 O 3 catalysts.   To evaluate the regeneration performance of the PtSn/3DOMM-Al 2 O 3 catalyst, a reaction activity test was carried out using a circulating reaction catalyst. As shown in Figure 11A, the initial propane conversion rate slightly decreased after the catalyst was first regenerated. However, in the second cycle reaction, the propane conversion rate was reduced by about 16% compared with the fresh catalyst. This may be due to the sintering of Pt nanoparticles after treatment with air at high temperatures; the increased selectivity is caused by the reduction of propene in reaction gas due to the decreased conversion ( Figure 11B) [45]. In Figure 11C the detailed changes in conversion, selectivity, and yield are listed in a bar graph. These results indicate that the propylene selectivity of the PDH reaction remained stable for the PtSn/3DOMM-Al 2 O 3 catalyst after regeneration. In summary, the 3DOMM-Al 2 O 3 as a good support improves both the physical and chemical properties of PtSn/γ-Al 2 O 3 catalysts, which can improve the catalytic performance of the PDH reaction.

Coke Analysis of Used Catalysts
The amount of coke was quantitatively obtained by TG-DTG, and the mass loss patterns are exhibited in Figure 12A. The total amount of coke follows the order PtSn/3DOM-Al 2 O 3 (3.9%) < PtSn/3DOMM-Al 2 O 3 (5.7%) < PtSn/M-Al 2 O 3 (16.6%). Generally speaking, in the DTG curve, two types of combustion peaks of coke will appear on the Pt-based catalyst. The lower temperature (around 460 • C) peak is assigned to the combustion peak of coke deposition on the around metal nanoparticles. The higher temperature (around 560 • C) peak is attributed to the carbon deposits on the surface of the support [46,47]. It can be seen from Figure 12B that the peak I and peak II appearing in the DTG curve correspond to the carbon deposition on the around metal (peak I) and the coke on the support (peak II), respectively. The peak I at 432 • C for PtSn/M-Al 2 O 3 can be considered as the coke deposited on the around Pt nanoparticle, and the peak II located at 546 • C for PtSn/3DOMM-Al 2 O 3 can be considered as the coke deposited on the Al 2 O 3 support, which can be attributed to macro-mesopore composite structure can facilitates the migration of carbon from the metal active site inside mesopore to the macropore in carrier. The PtSn/M-Al 2 O 3 catalyst has the largest amount of carbon deposits, which is consistent with the characterization of acidity. Because of it having more Lewis acidic sites on the M-Al 2 O 3 support, PtSn/M-Al 2 O 3 triggers more side reactions than PtSn/3DOMM-Al 2 O 3 . PtSn/3DOMM-Al 2 O 3 has moderate carbon deposits due to its suitable amount of acid sites. Notably, although PtSn/3DOM-Al 2 O 3 has a small number of carbon deposits, its catalytic performance for the PDH reaction is poor, thus the low amount of coke can be attributed to its low activity. In sum, the low carbon deposition of the PtSn/3DOMM-Al 2 O 3 catalyst can be attributed to the large surface area, low amount of acid sites, and highly dispersed Pt active components, which inhibits the carbon deposition. At the same time, the pore structure of the PtSn/3DOMM-Al 2 O 3 catalyst improves the carbon-holding capacity of the catalyst, which improves catalytic stability. port, PtSn/M-Al2O3 triggers more side reactions than PtSn/3DOMM-Al2O3. PtSn/3DOMM-Al2O3 has moderate carbon deposits due to its suitable amount of acid sites. Notably, although PtSn/3DOM-Al2O3 has a small number of carbon deposits, its catalytic performance for the PDH reaction is poor, thus the low amount of coke can be attributed to its low activity. In sum, the low carbon deposition of the PtSn/3DOMM-Al2O3 catalyst can be attributed to the large surface area, low amount of acid sites, and highly dispersed Pt active components, which inhibits the carbon deposition. At the same time, the pore structure of the PtSn/3DOMM-Al2O3 catalyst improves the carbon-holding capacity of the catalyst, which improves catalytic stability.  Figure 13 shows the Raman spectra of the used PtSn/Al2O3 catalysts. The Raman spectrum exhibits two peaks representing the degree of graphitization of the coke, namely 1330 and 1600 cm −1 . The Raman absorption peaks located at 1330 cm −1 and 1600 cm −1 are assigned to the D mode and G mode, respectively, which are caused by the multiple aromatic rings of graphite-like carbon materials. Among them, the D mode represents the disordered part of graphitized coking, while the G mode is considered to represent highly graphitized carbon. The ratio of intensity between the D and G modes is generally considered to be an important indicator for evaluating the graphitization degree of carbon material. Generally, a higher intensity ratio between the D and G modes means a lower  Figure 13 shows the Raman spectra of the used PtSn/Al 2 O 3 catalysts. The Raman spectrum exhibits two peaks representing the degree of graphitization of the coke, namely 1330 and 1600 cm −1 . The Raman absorption peaks located at 1330 cm −1 and 1600 cm −1 are assigned to the D mode and G mode, respectively, which are caused by the multiple aromatic rings of graphite-like carbon materials. Among them, the D mode represents the disordered part of graphitized coking, while the G mode is considered to represent highly graphitized carbon. The ratio of intensity between the D and G modes is generally considered to be an important indicator for evaluating the graphitization degree of carbon material. Generally, a higher intensity ratio between the D and G modes means a lower graphitization degree [48]. The ratio of the intensity of the D to the G peak (I D /I G ) was also obtained and is shown in Table 3. The ratio of I D /I G for the PtSn/3DOMM-Al 2 O 3 catalyst is 0.73, which is much higher than the ratios of I D /I G for the PtSn/M-Al 2 O 3 catalyst (0.66), indicating that the amount of disordered graphitized coke deposited on the PtSn/3DOMM-Al 2 O 3 catalyst is higher than that on the PtSn/M-Al 2 O 3 catalyst. graphitization degree [48]. The ratio of the intensity of the D to the G peak (ID/IG) was also obtained and is shown in Table 3. The ratio of ID/IG for the PtSn/3DOMM-Al2O3 catalyst is 0.73, which is much higher than the ratios of ID/IG for the PtSn/M-Al2O3 catalyst (0.66), indicating that the amount of disordered graphitized coke deposited on the PtSn/3DOMM-Al2O3 catalyst is higher than that on the PtSn/M-Al2O3 catalyst. It is widely known that the formation of coke during the PDH reaction involves three processes, while alkene is the main precursor of coke formation: (1) the deep dehydrogenation or cyclization of alkene; (2) the oligomerization of hydrocarbon; (3) a Diels-Alder type reaction [49]. The PtSn/3DOMM-Al2O3 catalyst has a lower amount of acid sites than the PtSn/M-Al2O3 catalyst, which inhibits deep dehydrogenation and cyclization by Lewis acid sites. Therefore, the PtSn/3DOMM-Al2O3 catalyst has a higher ID/IG value than the PtSn/M-Al2O3 catalyst. Notably, the PtSn/3DOM-Al2O3 catalyst has a much higher ID/IG value than the PtSn/3DOMM-Al2O3 catalyst. This may be due to the low partial pressure of propylene caused by low conversion of PtSn/3DOM-Al2O3 catalyst which inhibits the deep dehydrogenation process. For the PtSn/M-Al2O3 catalyst, a high amount of acid sites on the Al2O3 support resulted in the formation of more graphitized coke deposition. This also proves that the surface acidity of catalysts is an important factor affecting the anticoke stability of PtSn/γ-Al2O3 catalysts, which is also consistent with the conclusions of previous studies [50].    It is widely known that the formation of coke during the PDH reaction involves three processes, while alkene is the main precursor of coke formation: (1) the deep dehydrogenation or cyclization of alkene; (2) the oligomerization of hydrocarbon; (3) a Diels-Alder type reaction [49]. The PtSn/3DOMM-Al 2 O 3 catalyst has a lower amount of acid sites than the PtSn/M-Al 2 O 3 catalyst, which inhibits deep dehydrogenation and cyclization by Lewis acid sites. Therefore, the PtSn/3DOMM-Al 2 O 3 catalyst has a higher I D /I G value than the PtSn/M-Al 2 O 3 catalyst. Notably, the PtSn/3DOM-Al 2 O 3 catalyst has a much higher I D /I G value than the PtSn/3DOMM-Al 2 O 3 catalyst. This may be due to the low partial pressure of propylene caused by low conversion of PtSn/3DOM-Al 2 O 3 catalyst which inhibits the deep dehydrogenation process. For the PtSn/M-Al 2 O 3 catalyst, a high amount of acid sites on the Al 2 O 3 support resulted in the formation of more graphitized coke deposition. This also proves that the surface acidity of catalysts is an important factor affecting the anti-coke stability of PtSn/γ-Al 2 O 3 catalysts, which is also consistent with the conclusions of previous studies [50].

Conclusions
In this work, the effects of supports with different pore structures (3DOM-Al 2 O 3 , M-Al 2 O 3 , and 3DOMM-Al 2 O 3 ) on the PDH reaction performance of bimetallic PtSn-based catalysts were studied. The PtSn/3DOMM-Al 2 O 3 catalyst with a hierarchical porous structure shows the best catalytic performance among these three PtSn/Al 2 O 3 catalysts. the experimental characterization reveals that the macro-mesoporous composite structure of the PtSn/3DOMM-Al 2 O 3 catalyst has a significant impact on its physical structure and surface properties. The results indicate that the large specific surface area and hierarchical pore structure of the PtSn/3DOMM-Al 2 O 3 catalyst are beneficial to the dispersion of PtSn nanoparticles and gas diffusion from the inside of the mesopores. In addition, the difference in the pore structure of supports significantly changes the surface acidity and interaction between the metal and the support. In summary, the design strategy for the hierarchical porous γ-Al 2 O 3 support is an ideal method for improving the catalytic performance of PtSn-based catalysts in a PDH reaction.